The present invention relates to inkjet printers, including but not limited to large format printers, and to methods of operation thereof.
A problem with existing inkjet printheads, comprising a plurality of nozzles arranged substantially in a straight line, is that the nozzles at and adjacent the ends of the printhead or pen tend to eject inkdrops at an angle to a print medium rather than perpendicularly thereto as desired.
The Hewlett-Packard DesignJet 750 family of printers solved this problem by only using the central nozzles of the pen, thus avoiding the use of those with directionality problems. Since the extreme nozzles were not used at all, this produced clogging of these nozzles and deterioration of the reliability of the pen. In particular, since contaminants generally flow to the extreme nozzles actually being used, some of the central nozzles did not fire correctly. As the number of extreme nozzles with directionality problems increased, the portion of the pen actually being used had to become smaller and smaller to achieve satisfactory quality, which had an adverse effect on throughput.
European Patent Application no. EP 0 517 543, describes an inkjet recording method to control the unevenness of image density which is generated in the vicinity of a boundary between each recorded image area at each consecutive swath. This European Application is trying to address problems generated by form-feed advance accuracy and differences in drop volume (generating dot of different sizes) among extreme and central nozzles caused by manufacturing tolerances and heat distribution on nozzle plate. However it is not trying to solve problems generated by (i) swath height errors, i.e. nozzles firing at an angle which is different between central and extreme nozzles, or (ii) nozzle health, since the extreme nozzles are more sensitive to internal contamination, these are more likely to become clogged, misdirected or weak.
This helps to reduce the difference in optical density between a zone printed at 100% printing density in one pass and a zone printed at 100% printing density in two or more passes, irrespective of the distribution of the drops during the two or more passes. In particular dots made on the second pass are not printed in the position that would be desired for a maximum image quality, since this misalignment may produce wide lighter areas in the interswath regions.
Another previously-proposed solution was to print in two-pass mode throughout, but this produced essentially a doubling of printing time, which was not generally acceptable.
The present invention seeks to overcome or reduce one or more of the above-mentioned problems.
According to a first aspect of the present invention there is an inkjet printing apparatus having a printhead comprising a plurality of nozzles arranged along a printhead axis, the printing apparatus being arranged to print swaths which overlap the neighbouring swaths by a fraction of the swath width, characterised in that only one edge region of each swath which overlaps the neighbouring swaths has a reduced print density relative to its remaining regions.
Accordingly, this apparatus having only one edge region with a reduced print density may increase the throughput of the printhead.
According to a second aspect of the present invention, there is provided an inkjet printing apparatus having a printhead comprising a plurality of nozzles arranged along a printhead axis, the printing apparatus being arranged to print swaths which overlap the neighbouring swaths by a fraction of the swath width, the edge regions of each swath which overlap the neighbouring swaths have a reduced print density relative to their central regions said overlapping fraction at each edge characterised by lying in the range between one sixteenth and one quarter of the swath width.
Advantageously, this accurate selection of the overlapping region helps the apparatus improving the image quality of the printed image, by balancing the need of having a big overlapping region, which can make more pleasant to the eye the transition between the portion of the image printed in a one pass and the portion printed in a two pass, and a small overlapping region, which increase the throughput of the printhead.
According to a third aspect of the present invention, there is provided a method of operating an inkjet printing apparatus having a printhead comprising a plurality of nozzles arranged substantially along a printhead axis comprising the steps of printing swaths which overlap the neighbouring swaths by a fraction of the swath width, so that the combined density of the overlapping regions exceeds that of non-overlapping regions.